Getting a slab pour right

Bag counts are the easy part. These are the things that decide whether the slab is still flat in five years.

• Forgetting the base. Concrete poured onto soft ground will crack no matter how many bags you buy. Compacted gravel first.
• Mixing too wet. Extra water makes it easier to place and markedly weaker. Follow the water on the bag.
• Running out mid-pour. A cold joint between yesterday's concrete and today's is a permanent weak line. Have every bag on site before you start.

Rebar and control joints

#3 or #4 rebar on an 18 in grid suits most residential slabs, kept up off the ground on chairs so it sits in the middle of the pour rather than the bottom. Cut control joints at roughly 2.5 times the slab thickness in feet — about every 10 ft at 4 in. Concrete cracks; joints decide where.

Bag yields are not exact

Bag yields describe the finished volume of mixed concrete, not the dry weight, and they differ by brand by a few percent. A 5% margin absorbs that along with the low spots every real excavation has.